1. Field of the Invention
This invention relates to a butterfly valve that can be suitably used for pipe lines in chemical plants, water supply and sewage systems, agriculture and fishery. In particular, the invention relates to a butterfly valve in which a valve disc of the butterfly valve is displaced by fluid pressure when the valve is closed, leakage in the valve disc with the fluid leaking into the gap between the valve shaft hole and the stem of the valve disc is prevented while at the same time preventing fluid leakage from the shaft seal part between the stem and the through holes of a seat ring, and wherein the operation torque of the stem is not increased and no fluid leaks from the shaft seal part or the sealed part between the valve disc and the neighborhood of the through holes of the seat ring or the downstream side of the valve after continuous on/off (open/close) operation over a long period of time.
2. Description of the Related Art
A conventional butterfly valve of resin, as shown in FIG. 8, comprises a hollow cylindrical valve body 51, an annular seat ring 52 fitted to the inner peripheral surface of the valve body 51, a stem 53 supported on the valve body 51 through the seat ring 52 and a valve disc 55 supported by the stem 53 in a valve shaft hole 54 through the stem 53, wherein the valve disc 55 is rotated by the rotation of the stem 53 and pressed against or separated from the seat ring 52 thereby opening/closing the valve. In the process, with the increase of the bore diameter of the resin butterfly valve, stress is concentrated mainly on the intermediate portion in the axial direction of the stem of the valve disc 55 by the fluid pressure exerted with the valve closed, with the result that the displacement is caused from the intermediate portion (FIG. 9). This displacement undesirably separates the seat ring 52 and the outer peripheral edge 58 of the valve disc 55 on the upstream side (right side in FIG. 9) which otherwise normally would be in contact with each other under pressure. Thus, a gap 59 is generated (FIG. 10), and fluid may leak into the gap 60 between the valve shaft hole 54 and the stem 53 inside the valve disc. On the downstream side (left side in FIG. 9) free of the fluid pressure, a seal is formed with the valve disc 55 pressed strongly against the seat ring 52, and therefore, no fluid leaks from the valve even if leakage inside the valve disc occurs. Depending on the fluid, the stem 53 may corrode. Especially in the case where the fluid is corrosive which corrodes the metal stem 53 and may deteriorate the strength and endurance or destroy the valve. In the case where the shaft seal part 56 between the seat ring 52 and the stem 53 is not sufficiently sealed, the fluid leaks from the shaft seal part 56 after continuous operation under a high fluid pressure or over a long period of time. As a result, the fluid intrudes into the part 57 behind the seat ring, i.e. between the outer periphery of the seat ring 52 and the inner periphery of the valve body 51. Further, the stagnation of the fluid expands the seat ring 52 radially inward, thereby posing a problem in that operation torque is increased or the valve disc 55 cannot be closed.
In the case of a metal butterfly valve, though not to the extent of a resin butterfly valve, the valve disc 55 of a larger bore diameter is displaced more by the fluid pressure exerted during the closed state, and therefore, the leakage in the valve disc may occur from the gap 59 between the valve disc 55 and the seat ring 52. Also, like in the resin butterfly valve, the fluid may leak from the shaft seal part 56 between the seat ring 52 and the stem 53. Further, although the valve disc 55 of the metal butterfly valve is lined for use with a corrosive fluid, the portions into which the fluid leaks are not normally lined. Thus, the corrosive fluid corrodes the metal stem, thereby posing a problem that the strength and endurance of the valve may be deteriorated or the valve may be damaged.
As a solution to the problem of fluid leakage from the shaft seal part posed in the prior art described above, a shaft seal device of the seat ring is described in Japanese Patent No. 3389542 (pp. 1-3, FIG. 3). In this central-type butterfly valve, as shown in FIG. 11, a seat ring 62 of an elastic material is attached to the whole inner peripheral surface of a valve body 61 through which a hollow cylindrical fluid path is formed, and a discal valve disc 63 with the outer peripheral surface thereof adapted to come into or out of contact with the seat ring 62 is rotatably journaled by a valve rod 64. This central-type butterfly valve is characterized by an expanded portion 65 integrated with the seat ring 62 and extended from the inner peripheral surface of the seat ring 62 in parallel to the axial line of the valve rod 64 of the seat ring 62 radially inward of the valve body 61 without insertion of a rigid member such as a ring, and an expanded portion 66 integrated with the seat ring 62 and extended inward from the inner peripheral surface of the insertion hole for the valve rod 64 in the direction orthogonal to the axial line of the valve rod 64. The advantage of this central-type butterfly valve is that fluid leakage along the axial line of the valve rod 64 can be positively prevented by increasing the compression rate of the expanded portions 65, 66 as compared with the other parts.
In the shaft seal device for the seat ring described above, the valve rod 64 and the seat ring 62 are sufficiently sealed by the expanded portions 65, 66, and therefore, fluid leakage in the direction along the valve rod axis between the valve rod 64 and the seat ring 62 is prevented. In the case where the valve disc 63 is displaced by fluid pressure exerted when the valve is closed, as shown in FIG. 11, the sealing capacity of the expanded portion 65 is insufficient, so that a gap develops between the seat ring 62 and the valve disc 63 and the fluid may leak into the mounting portion of the valve rod 64 and the valve disc 63 from the gap in the valve disc 63 (hereinafter referred to as the valve disc internal leakage). This can be improved by enlarging the expanded portion 65. However, an increased size of the expanded portion 65, makes it difficult to assemble the valve and poses the problem of increased torque of the valve rod 64. Also, the expanded portion 65 is kept strongly deformed between the valve disc 63 and the valve body 61. Although no problem occurs over a short period of time, continuous opening/closing (on/off) in a protracted use may buckle the deformed expanded portion 65, resulting in lower sealability. Thus, the fluid may leak downstream, or internal leakage may occur in the valve disc, thereby leading to the problem that the valve rod 64 may corrode.